1. Field of the Invention
The present invention generally relates to an apparatus for and a method of measuring the surface shape of an object, and more particularly, to the measurement of the surface shape of an object that includes a step of a height greater than ½ of the wavelength.
2. Description of the Related Art
A laser interferometer is known as an apparatus for measuring the surface form of an object in the range of the wavelength of light. For example, a laser interference system manufactured by Zygo Corp. is available in the market. The laser interferometer takes a plurality of interference fringe images using an imaging device such as a CCD with a reference beam of which phase is modulated, obtains the phase of reflected beam from the object based on the interference fringes, and obtains the shape of the object based on the phase data at the range of the wavelength of the reference beam.
However, if the object includes therein a step of more than ½ the wavelength, the step makes the unwrapping of the phase impossible and causes a phase discontinuity, and the result of the measurement consequently includes an uncertainty.
A scanning white light interferometer, a 3-D surface profiler of Zygo Corp., for example, solves the problem by expanding the measurement range λ/2 in the directions of the beam axis for a single wavelength based on the correlation phase data measured using a plurality of wavelengths (multiple wavelengths measurement). In the case where the step is greater, one measures absolute length of the object by scanning either the interferometer or the object in the directions of the beam axis and detecting the positions of the white interference fringes and the moving length of the interferometer or the object since white interference fringes appear only when the light path lengths of the reference beam and the reflected beam from the object are substantially equal.
However, in the case where a user measures a large step with the scanning white light interferometer, the measurement takes a long time because the user must move either the object or the interferometer in the directions of the light axis. Additionally, since the user must move the object or the interferometer during the measurement and measure the moving distance of the interferometer or the object, more error in measurement may be involved. The range of measurement in the light path directions is limited by the movable range of the interferometer or the object.
Japanese Laid-open Patent Application No. 2000-221013 discloses an invention of a scanning white light interferometer that reduces the interference fringe images in number and consequently speeds up the measurement so that the user can measure the shape of an object including steps and absolute lengths greater than the wavelength with high precision and at high speed without making a mistake.
Japanese Laid-open Patent Application No. 2001-41724 discloses an invention of an apparatus for measuring surface shape using an interferometer that employs the phase shift method using birefringence so that the user can measure the 3-dimensional shape of the surface of a measurement object with high precision without moving the measurement object. If an electro-optic component is used instead of a phase plate to shift the phase of the reference beam, the user can speed up the measurements.
Moreover, the method based on the depth from focus theory is a technique to obtain the shape of an object based on the intensity data obtained by measuring the object by a CCD. One obtains a position at which the light goes into focus based on a plurality of images taken while changing the focal length, and obtains the surface shape of the object. (Refer to “High-speed 3-dimensional measurement of shape by the depth-from-focus method” Journal of Japan Society for Precision Engineering, vol. 63, No. 1, 1997, M. Ishihara, H. Sasaki, for example).
However, the technique disclosed in Japanese Laid-open Patent Application No. 2000-221013 still includes a mechanical portion therein since it employs the phase modulation method (phase shift method) to measure in the sub-fringe order and requires taking a plurality of images. Accordingly, this technique can reduce the time required for measurement to some extent compared with the scanning white light interferometer, but it is considered that this technique cannot speed up the measurement beyond a certain limit. This technique is not applicable to the measurement of dynamic change in shape of a moving object. Additionally, the problem that the mechanical aspect of this technique increases error factors in the measurement still remains.
The technique disclosed in Japanese Laid-open Patent Application No. 2001-41724 is basically the measurement of the interference of a laser beam based on the phase shift method. As described above, this technique cannot measure a surface including a step of more than ½ wavelength due to uncertainty. Additionally, this technique also speeds up the measurement to some extent, but it still requires taking a plurality of images. Accordingly, this technique cannot reduce the time required for the measurement and consequently is not applicable to dynamic measurement of moving shapes.
Moreover, the technique based on the depth-from-focus theory is generally not suitable to the measurement of an object having shapes of sub-fringe order since the resolution in measurement of this technique is lower than that of the laser interference. It is possible to improve the resolution to a certain extent by increasing the magnification of the optical system, but the transverse range of measurement (field of view) is reduced if the resolution is increased up to the same level as the laser interference. Since this technique requires taking a plurality of images as the focal length is changed, it requires at least the time to take the plurality of images, whatever method is used to take the images. Because this technique also requires a precise mechanism to adjust the focal length of a lens, the structure of the equipment becomes complicated. The mechanical portion of the equipment increases error factors and the measurable range in the directions of the beam axis is limited by the movable range of a moving portion of the equipment.